GeezerVison

The result of EXPLAIN QUERY PLAN is a flat grid of (id, parent, notused,
detail) rows, which makes the nesting of the plan hard to see. Add an
ExplainTree helper that derives the nesting depth of each plan step from
the id/parent columns (robust against out-of-order rows, unknown parents
and parent loops) and an item delegate which shows the detail column
indented by that depth. The Execute SQL tab installs the delegate only
when the executed statement is EXPLAIN QUERY PLAN; the model data, copy
and export are unchanged and other results are not affected.

Includes the test-explain-tree unit test.